The invention relates to isocyanate-functional polyurethane prepolymers as well as to aqueous dispersions thereof and the use thereof for the production of flat materials.
In the early 1990's, general prophylactic measures adopted world-wide against HIV, hepatitis B and hepatitis C led to dramatically increased use of latex-based gloves and condoms. Approximately 1% of glove users and approximately 10% of users working in the health field showed allergic sensitization reactions, and this has over recent years considerably increased the importance of the quest for alternative materials in the examination glove and condom sector.
Natural latex contains type I and type IV allergens. Type I allergens are attributable to the proteins which naturally occur in latex, and can even result in anaphylactic shock. Type IV allergens are the accelerators and additives needed in latex production. These frequently lead to hypersensitive contact dermatitis. The allergic reactions described are not known to result from polyurethanes. There is therefore a pressing need for aqueous polyurethane dispersions which can be processed analogously to rubber latex to obtain the aforementioned hygiene articles.
According to the teaching of WO-A 00/61651, aqueous polyurethane dispersions obtained from nonionic isocyanate prepolymers after the addition of anionic emulsifiers and subsequent dispersion with water are suitable for the production of gloves and condoms. A disadvantage of these products, however, is the use of the anionic emulsifiers, which may migrate from the polymers in service, with a consequent negative effect on the service characteristics. The same limitations apply to the products obtainable according to the teaching of WO-A 00/61653.
A major problem when polyurethane gloves are used in medicine and in particular in surgery is the use in these applications of isopropanol-containing mixtures for sterilizing the gloved hands. The utilization of isopropanol brings about swelling of the glove material, which greatly reduces the mechanical resistance of the material, resulting in tearing of the gloves.
U.S. Pat. No. 5,997,969 describes a way of improving the isopropanol resistance of normal commercial polyurethane dispersions by the post-addition of cross-linking components. The addition of a cross-linking component is, however, a technically very demanding and costly process for manufacturers of gloves and condoms, because the mixing devices needed for this purpose are not normally available. Such mixtures of reactive cross-linking agent and substrate have only a limited pot life. This is also a problem for which there can be no ready solution for glove and condom manufacturers, because processing normally follows the coagulation operation, and coagulation batches are not normally used up. They are generally replenished repeatedly with fresh material.
European patent 741,152 discloses NCO-functional polyurethane (“PU”) prepolymers based on a polyol component which principally comprises polyoxypropylene diols having low unsaturated unit contents. The dispersions prepared from these are suitable for the production of flexible films and coatings. It is, however, disadvantageous that the PU films have unsatisfactory resistance to solvents.
Canadian patent 1,089,141 describes mixtures of aromatic and aliphatic or cycloaliphatic polyisocyanates for the preparation of finely divided, stable aqueous dispersions of anionically modified polyurethanes. However, the resistance to isopropanol of the flat materials which can be produced from them is likewise deficient.
U.S. application Ser. No. 10/453,755, filed on Jun. 3, 2003 describes several additional references which are directed to the use of polyurethane dispersions for medical applications. The inventions described in the 10/453,755 application are an isocyanate functional prepolymers, aqueous polyurethane dispersions produced from the prepolymers and various medical uses of such dispersions. The prepolymers described therein have an NCO content of from about 1 to about 6% by weight, and are prepared by reacting:                A) an organic diisocyanate,        B) at least one dihydroxy compound having a number average molecular weight of from about 700 to about 16,000, and        C) a trihydroxy component of the formula:R—(OH)3         where R is a saturated straight chain or branched chain aliphatic group of from 2 to 8 carbon atoms, andwherein the amount of component C) is such that the hydroxy groups from component C) amount to from about 2 to about 15% based on the total amount of hydroxy equivalents used to produce the prepolymer.        